Vapor generating and superheating method and apparatus



March 13, 1956 w, H. ROWAND ETAL VAPOR GENERATING AND SUPERHEATING METHOD AND APPARATUS Filed April 16 1949 2 Sheets-Sheet 2 United States Patent() VAPOR GENERATING AND SUPERHEATING METHOD AND APPARATUS Will H. Rowand, Madison, N. J., Arthur E. Raynor, Rockville Centre, N. Y., and Charles H. Wooliey, Cranford, N. 1., assignors to The Babeock & Wilcox lilompany, Rockleigh, N. J., a corporation of New ersey Application April 16, 1949, Serial No. 87,882

9 Claims. (Cl. 122-479) This invention relates to a method of generating and superheating a vapor under pressure.

The method is exemplified in the operation of a radiant type steam generator unit in which a preponderance of the steam generation is accomplished by the radiant heat transmission to steam generating wall tubes in an intermediate zone, while the superheating and reheating of the generated steam is accomplished by convection superheating and convection reheating surfaces subject to the flow of heating gases from said zone.

' In the attainment of high thermal efiiciency in steam prime movers, recent trends have involved the use of steam generator pressures within the range of 1500-2500 p. s. i. with the steam superheated to temperatures within a 950-l100 F. range for expansion through a high pressure steam turbine to a pressure within the 400-600 p. s. i. range at which it is again subjected to convection reheating to a temperature of the order of 1000 F. With such high pressures and temperatures there has been a marked increase in ratio of heat required for superheating a pound of steam to that required for its generation from water at saturated temperature. When reheating of the steam is also involved the ratio of the heat which must be absorbed in superheating and reheating steam to that required for steam generation is still further increased.

To fully realize the optimum economics of high pressure and hi h temperature steam supply to a steam turbine prime mover, it is necessary to be able not only to supply steam at constant pressure regardless of load changes, but also to attain the delivery of high pressure superheated steam and the lower pressure reheated steam at substantially uniform temperatures irrespective of load changes.

As the preponderance of steam generation results from radiant heat transmission to vapor generating wall tubes and is a function of the heat radiating temperature of the furnace gases, there is usually little variation in the heat absorption inasmuch as furnace gas temperatures do not vary widely with variation of rating in the customary method of furnace operation. Thus when heating gases of adequate quantity and temperature are delivered to a convection superheater and a convection reheater for attain ment of the optimum degrees of superheat and reheat at high loads, it will be found that when the load and the fuel input are reduced, the furnace heat absorption will still be such that insufiicient heat remains in the gases passing from the furnace to the superheater and reheater to attain the desired degrees of superheating and reheating even though the steam flow has decreased.

It is an object of the invention to provide a method of vapor generation and superheating whereby regulation of heat input into the fluid cooled vapor generating walls may be effected, and coordinated with variation in rates of fuel delivery or vapor generation by the unit so that the heating gases flowing from the furnace will carry sufiicient available heat to superheat and reheat the steam flow to the optimum degree.

To accomplish this object the invention involves the 2,737,930 Patented Mar. 13, 1956 ice reduction of heat absorption in a radiant vapor generating zone, or stage, with reduced loads, to an effective degree by effecting a reduction in the radiating temperature of the heat carrying gases, by the introduction of recirculated gases from the gas pass at a position rearward of the superheater and reheater surface into the gas inlet of the radiant zone of vapor generation, and by the thorough mixing of the recirculated gases with the other combustion products in said zone. The invention simultaneously involves a relative increase in heat absorption by the superheater and reheater by reason of the increased gas mass flow.

More specifically, the invention involves the eXtrac tion of some of the combustion gases from a zone on the downstream side of the superheater, and the passing of those gases to the high velocity inlet of the radiant zone of vapor generation wherein they are mingled and thoroughly mixed with the combustion products from a combustion chamber at a position of high gas velocity and high turbulence. The effect of this operation is to decrease the adiabatic temperature of the gases in the radiant zone of vapor generation, (or the radiant gas cooling the ash fusion temperature of the fuel involved, and a large volume gas cooling chamber receiving the combustion products from the cyclone furnace. The furnace gases pass from the gas cooling chamber to a convection section including a superheater and a reheater. From the downstream side of superheating surface, furnace gases are recirculated to a position adjacent the gas outlet ot the cyclone furnace, where high turbulence exists. This condition promotes a thorough mixing of the recirculated gases and the combustion products passing into the gas cooling chamber and this mixing, in turn, minimizes laning of the gases, and promotes uniform heat absorption. The illustrative installation may be regarded as including a primary furnace chamber having a slag tap bottom covered with a layer of molten slag during the operation of the unit.

The combination of the cyclone furnace, or cyclone type of burner for the combustion of slag forming solid fuels with an arrangement for effectively mixing recirculated cooled gases from a position rearwardly of the vapor superheating zone is particularly advantageous in that high combustion and ash separating efliciency is attained by the cyclone while a subsequent depression of the temperature of the resulting mixture of the primary products of combustion and the recirculated gases may be quickly attained. I

The cyclone furnace also operates under positive or super-atmospheric pressure, and this promotes high gas velocities in the outlet zone of the combustion stage which may also be regarded as the inlet zone of the radiant gas cooling and vapor generating stage. This is a condition which definitely enhances the thorough mixing of the recirculated gases with the gases and other combustion products from the furnace.

Long flame burning with consequent Stratification of gases of different temperatures and different amount of unburned combustible constituents flowing through the radiant gas cooling is avoided.

The invention is described herein with reference to g a unitary installation shown in the accompanying drawings, and further objects of the invention will appear as the description proceeds.

In the drawings:

Fig. 1 is a vertical section of the illustrative installation; and

Fig. 2 is a horizontal section on the line 2-2 of Fig. 1.

The illustrative method is effected by the installation indicated in the drawings. This installation is a steam power plant of 830,000 lbs of steam per hour capacity at 1850 p. s. i. and at a superheat of 1010 F. It reheats steam at 730,000 lbs. steam per hour capacity at 550 p. s. i. from 690 F. to 1010 F.

In this installation combustion is effected in a cyclone furnace such as that indicated at in Fig. 1. This furnace is characterized by high combustion eificiency and a recovery of a preponderant proportion (90-95%) of the recoverable portion of the fuel ash as molten slag.

The cyclone furnace and the closely associated components are constructed and arranged in a manner similar to that indicated in the U. S. patent to Bailey et al. 2,357,301, September 5, 1944. In the operation of the furnace, crushed coal and a supply of preheated primary air under super-atmospheric pressure are delivered tangentially to the cyclone burner inlet 12. This introduction of primary air and coal into the furnace takes place at a velocity high enough to cause the coal particles to be thrown toward the cylindrical wall of the cyclone formed by fluid cooled studded tubes, covered on their inner sides by high temperature refractory material. High temperature secondary air is also admitted to the cyclone in a path generally parallel to that of the primary air and coal. Such secondary air is directed to the cyclone through the duct work 16. The temperature of the products of combustion in the cyclone furnace is high enough to melt the ash into liquid slag which clings to the wall of the furnace. The incoming coal is trapped in this surface layer of molten ash, and the scrubbing action of the high velocity combustion air over these coal particles results in substantially complete combustion at high burning rates, with a minimum of excess air.

The combustion gases leave the gas outlet 17 of the cyclone in a highly turbulent condition and slag formed in the cyclone accumulates to such an extent that it flows through a slag opening 18 formed by outward bends such as 19 of some of the tubes defining the rearward wall 20 of the cyclone.

Slag flowing from the cyclone 10 through the opening 18 drops to a slag pool on the bottom 21 of the primary furnace chamber 22 from which it passes through an opening 23 and drops into a body of water within the slag tank 24.

The upper and rearward wall of the primary furnace chamber 22 is defined by steam generating tubes connected into the circulation of the installation, parts. of these tubes defining the reflecting arch 25 which is faced with refractory on its furnace side. Below this arch the same steam generating tubes are bent to form the transversely spaced platens of a slag screen 26, extending at an angle of approximately 45 to the horizontal.

These platen forming tubes extend downwardly of the unit to connections with the lower drum 116, and above the wall 25 and the bends 25a; these same tubes extend vertically from the boundary surface 109 to connections with the upper drum 118. Between these positions the pertinent sections of these tubes are arranged in the form of horizontally spaced platens extending into chamber 28 from its wall 108. This construction promotes increased radiant heat absorption within the secondary furnace chamber 28. One of such platens may be considered as indicated at 26a.

The gases from the primary fvurnace chamber 22 flow through a zone 27 which will be: described later. From that zone the gases flow upwardly through the radiant gas cooling chamber 28 and then turn to the right and flow horizontally across the elements of the secondary superheater 29 and the reheater 30.

Rearwardly of the reheater, the gases turn downwardly in the equalizing chamber 31 at the top of upright gas pass 32. Proceeding through this pass, they first flow over the tube banks 3437 constituting the primary (or intermediate) superheater 38. The gases then flow over the bank of tubes constituting the economizer 40.

Beneath the economizer 40, there are two transversely spaced air heater sections 42 and 44 (Fig. 2) each section consisting of a bank of upright tubes connecting upper and lower horizontal tube sheets. These sections are spaced apart horizontally to provide between them the inlet of a gas bypass duct having front and rear walls 5254, respectively, and leading to the inlet 56 of the fan 58. The outlet 60 of this fan is connected to duct work 62-64 providing a passage leading to the ports 66 through which the recirculated gases are caused to enter the inlet zone 27 of the radiant gas cooling chamber 28.

Gases not recirculated to the secondary furnace chamber 23 pass through the breeching 70 in the direction of the arrow 72 to the inlets of the air heater sections 73 and 74 from which the gases flow to an appropriate flue. Air for combustion enters the air heater inlet as indicated by the arrow 80, proceeds over the tubes of the successive air heater sections, and then passes through ductwork 16 to the air inlet 75 of the cyclone furnace 10.

The radiant gas cooling chamber or secondary furnace chamber 28 is of rectangular horizontal cross-section, its boundary surfaces such as 104109 being defined by rows of vapor generating tubes connected into the circulation system of the installation by tubular connections at their lower ends with the submerged drum 116, and similar connections at their upper ends communicating with the steam and water drum 118. Some of the tubes thus connected have portions defining the forwardly inclined lower Wall of the superheater and reheater gas pass. Other sections 122 of these same tubes extend across the gas pass leading to the gas turning and equalizing chamber 31, and succeeding sections define the roof 124 above the reheater 30 and the secondary Superheater 29.

The gas pass 32 leading downwardly from the chamber 31, and containing the economizer 40 and the superheater tube banks 34-37, has its rearward wall and its roof 132 at least partly defined by superheater inlet tubes, upright portions of which are indicated at 134. Some of these tubes extend along the gas pass roof 132 to the inlet header 136 which is supplied with steam from the drum 118 through tubes such as 140. Other upright steam inlet tubes such as 142 extend vertically above the wall 130 and then horizontally as indicated at 144, to the header 136.

The wall of the gas pass 32 opposite the wall 130 is defined by similar superheater inlet tubes connected to the intermediate superheater header .152 at their lower ends, and having their upper ends connected to the inlet header 136. Steam flows from the header 152 through tubular connections such as 154 to the lowermost bank of superheater tubes 37. The header 152 is U-shaped, having side wall parts, such as 153, connected by upright side wall tubes 155 to upper headers, such as header 157, directly connected to drum 118 by tubes 159. The flow is continued through series connections between tubes of the various superheater banks to the intermediate superheater header 156. From this header the superheated stearn flows through tubular connections 158 to superheater attemperators 160 and 162. From theattemperators, the steam flows through tubular connections 164 and 166 to the inlet header 168 of the secondary superheater 29. This superheater consists of a plurality of series connected return bend tubes as indicated in the drawing, having their outlet ends connected to the outlet header 170 from which the superheated steam flows through a conduit 171 to a point of use, such as a turbine.

Steam from the turbine flows through the reheater attemperator 173 and then through a connected tube 174 to the reheater inlet header 176. The flow then continues through the return bend sections of the reheater 30 to the outlet header 178, and thence through appropriate tubular connections to a point of use.

It is to be understood that many parts of the cyclone 'furnace, the radiant gas cooling chamber and other parts of the installation, are defined by steam generating tubes such as those which are line indicated as connected to headers such as 261-205, which are appropriately connected into the fluid circulation of the system by connections at their lower parts with the submerged header 116 and appropriate connections at their upper ends with the steam and water drum 118.

The illustrative installation is designed for maximum fuel input and optimum sup-erheat at full load. Superheat heat regulation from 100% load to 65% may be efiected by attemperators such as 'those indicated at 160 and 162, constructed and arranged as is well known in the art. Reheat regulation over a similar load range may take place in a similar manner through the intermediacy of the reheat attemperator 172.

To provide sufficient heat for superheat and reheat re: quirements at still lower loads and to so simultaneously and oppositely affect the operation of the steam generating surface, on the one'hand, and the combined reheating and superheating surface, on the other hand, that predetermined superheat temperature is maintained over a wider load range (i. e., 65-50%), the invention involves the passage of combustion gases from a position downstream of the economizer 4t) and into the furnace through the ports 66 in the manner above indicated. These recirculated gases are mixed with the gases from the cyclone furnace in the manner hereinafter described. This action results in a decrease in the heat absorption rate of the steam generating tubes forming the Walls of that chamber, as compared to the heat absorption rate, when no recirculation is employed. As the preponderance of the steam generated in the installation results from radiant heat transmission to the walls of the chamber 28, and is a function of the heat radiating temperature of the furnace gases, there is little variation in the heat absorption of the furnace walls inasmuch as furnace gas temperatures do not vary widely with variation of rating, in the customary method of furnace operation. By the use of this invention, however, the heat absorption by the steam generating tubes forming the walls of the chamber 28 is lowered at reduced loads by effecting a reduction in temperature of the heat radiating gases. This is accomplished in a particularly effective manner in which the introduction of recirculated gases from the gas pass at a position downstream of the superheater and reheater surface into the inlet zone or passage 27 through the ports 66 is accomplished as hereinafter described.

The relation of the cross-sectional flow area of the inlet zone 27 to the flow area between the spaced platens which extend from the lower edge of the depending refiecting arch to the bottom 21 is such that the gaseous products of combustion flow through zone 27 with a continuation of the high velocity with which they emerge from the secondary furnace chamber 22. No material diminution of velocity will be experienced until the gases rise into the chamber 28, with its walls upwardly diverging from the level of tube bends 25a. As the spaced platens 26 extend at an angle of approximately 45 to the horizontal, the high velocity gases flowing through the spaces therebetween will be directed into zone 27 at an angle approaching 45 to the horizontal and there- 6 fore upwardly inclined toward the position of the 66.

The front to back dimension from tube bends 25a to the vertical rear wall 105 is considerably less than the corresponding front to back dimension of chamber 28, above the inlet zone 27. Downwardly inclined recirculating gas ports 66 are positioned at substantially the same level as tube bends 25a so that the high velocity gas jets of recirculated gas are directed downwardly therefrom into the high velocity stream of high temperature combustion gases flowing from the secondary fur nace chamber 22, insuring that the penetration of the jets will be most efiective in promoting turbulence and mixing of the gases before the gaseous mixture leaves zone 27, and before the velocity is materially reduced not only by the reduction in temperature due to the admixture of the recirculated gases but also as the result of the diverging section of chamber 28.

The recirculated gases in the cited example will be at a temperature of the order of 600 F. and the fact that the high velocity jets of such gases which will be materially denser than the high temperature gases emerging from secondary furnace 22 is a factor insuring good penetration and mixing.

The above described arrangement of recirculated gas nozzles results in a quick and effective mixing of the recirculated gases with the primary combustion gases to insure a rapid reduction in temperature of the mixture which will emerge from zone 27, and which upon further reduction in velocity in the expanding lower portion of chamber 28 will radiate heat to the water cooled walls of that chamber.

With the use of the recirculated gases, the temperature of the gases contacting the reheater and superheater elements will be at a lower value than would be the case without recirculation. However, the increased amount of gases contacting said convection surface and resulting from said recirculation, results in increased mass flow of the gases to an extent which more than compensates for the decreased temperature or" the gases at the outlet of the chamber 28. The net result is such an increase in the percentage of heat absorbed by the combined supereater-reheater components, and simultaneously such a decrease in the heat absorbed by the steam generating elements that superheat is maintained at an optimum value, i. e. 1050 F. as load varies from -50% of maximum, or below. It will be understood, of course, that as the load is reduced, the percentage of gases extracted from the gas pass 32 at a position downstream from the economizer 4t) and recirculated to the chamber 28 at the inlets 66, will be increased, and vice versa.

The entire vapor generating installation is under a positive, or superatmospheric pressure; no induced draft fan is utilized; and the delivery pressure of the secondary air fan is sulficient to overcome the pressure loss through the cyclone burner, in addition to the pressure loss through the gas cooling chamber 28, and in addition to the pressure loss through the subsequent convection section or sections, up to the stack. The recirculated gas fan provides a delivery pressure sufficient to overcome any pressure in the high velocity inlet zone 27, and gives a delivery velocity in this zone of a value sufiicient to provide the penetration and mixing action by which the recirculated gases and the other high velocity combustion gases from the cyclone burner are thoroughly mixed.

The drawing indicates a diagrammatic system for effecting such control. It involves a fan motor 200, the speed of which is varied in response to the effect of variations in superheat temperature upon the thermally responsive device 212 disposed in the superheater outlet conduit 171. This device is effective, through a line 206 upon motor speed control mechanism 208 connected as i at 210 to the motor 200.

By way of recapitulation of the invention, it involves ports the steam generation and the superheating is attained'by cyclonically efiecting'the combustion of a particle form slag forming fuel'while maintaining a normal cmbus-' tion zone temperature higher than the slag fusion temperature. The fuel is burned in a cyclone furnace'arranged with its major axis substantially horizontal and having a gas outlet opening into the lower portion of a vertically elongated gas cooling furnace chamber having a substantially horizontal floor for slag collection and continuous removal. The fused slag or ash particles, and the combustion gases are separately discharged from the high temperature combustion zone, and the fused slag or ash particles are collected in the stratum for continuous removal as molten slag. For attaining these results, the cyclone furnace has'a gas outlet opening into the lower portion of the gas cooling chamber, and there is a slag discharge opening or passage leading from the cyclone furnace into the gas cooling chamber below the level'of the cyclone furnace gas outlet and above the level of the slagcollecting floor. The floor is formed with a slag discharge outlet so that there may be continuous removal of slag from the flowing stratum on the floor. The invention also involves the generation of high pressure steam or vapor and the simultaneous cooling of the combustion gases predominantly by the radiant transmission of heat from the gases to confined streams of a vaporizable liquid. To attain these results vapor or steam generating tubes are disposed in or along one or more walls of the vertically elongated gas cooling chamber, the transmission of heat from the high temperature gases in the gas cooling chamber substantially reducing the temperature of the gases flowing therethrough.

In addition to the subject matter of the immediately preceding paragraph, the invention involves the superheating of the generated vapor or steam by convection heat transfer from the combustion gases beyond the main zone of vapor or steam generation. Such superheating is effected by convection heat transfer to a bank of steam or vapor superheater tubes externally subject to gas flow from the gas cooling chamber.

In a more specific sense, the invention also involves regulably increasing the convection superheating effect of the superheater tube tank by variably withdrawing a portion of the combustion gases at a point in the gas flow path beyond the superheater tube tank, and returning the gases to the lower portion of the gas cooling chamber at a point in the gas fiow path therein nearer to the gas outlet of the combustion zone than to the convection superheating zone, the recirculated gases being discharged into the radiantly heated vapor generating zone of the gas cooling chamber at such a position that the recirculated gases introduced in this zone are separated from the stratum of fused slag in the slag discharge zone, by a body of the higher temperature combustion gases. This action is effected by gas recirculation systemhaving a gas inlet communicating with the combustion gas flow beyond the superheater, and having a gas outlet at a position beyond the cyclone furnace and remote from the slag stratum upon the floor of the vertically elongated gas cooling chamber.

In combination with the subject matter indicated, immediately above, the invention involves causing the high temperature combustion'gases from the combustion zone to sweep across the stratum of the fused slagin theslag discharge zone, and to sweep across the slag discharge outlet of that zone before these gases meet the oncoming recirculated gases.

In a still more specific sense, the invention involves V the above indicated combination with the gas recirculationsySt'em acting' as a part of superheat control means constructed and arranged to control 'superheat'over a wide range of rate of steam orvaporgenerationbY increasing the percentage of gases recirculatedas the rate 7.7

of steam generation decreases, to maintain the temperature'of-the superheated 'vaporor steam, at a predeten mined value.

Specifically, the invention furtherinvolves the control of 'the'temperature of superheated vapor steam by the introduction of returnediand lower temperature combustion gases in such. a manner that there is no such heat transfer from the fused slag stratum, upon the floor of the slag discharge zoneto the recirculated gases that the slag is cooled below its fused or liquid state. This manner of operation involves thecausing of the high temperature combustion, gases, at a temperature -very much above the slag fusiontemperature to sweep across the stratum of fused slag upon the bottom of the slag disposal zone and to sweep across the slag discharge outlet before they meet the oncoming recirculated gases of lower temperature. The means for effecting this result includes a division wall formed by some of the steam or vapor generating wall tubes of the gas cooling chamber, these tubes defining a division wall separating the gas cooling chamber into two component parts, one of which may be termed a primary furnace chamber and the other a .secondaryfurnace chamber. The gas communication between these. two chambers is at the lower part of the division wall so that the high temperature gases from the outlet of the combustion zone are compelled to proceed downwardly toward the slag removal zone, and then upwardly around the lower end of the division wall before these high temperature gases meet the oncoming recirculated gases at lower temperature.

While in accordance with the provisions of the statutes we have illustrated and described herein the best form of our invention now known to us, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by our claims, and that certain features of our invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. In a method of vapor generation and vapor superheating; effecting the combustion of an ash bearing fuel at'a temperature above the fusion temperature of the ash and under highly turbulent conditions; separating the fused ash particles from the combustion gases; collecting the fused ash particles in a stratum for continuous removal as molten slag; generating a high pressure vapor and simultaneously cooling the combustion gases predominantly by the radiant transmission of heat from the gases to vaporize confined streams of a vaporizable liquid; superheating the generated vapor by convection heat transfer from the combustion gases beyond the zone of vapor generation; effecting optimum superheat at a predetermined rate of vapor generation; and increasing, at fractional loads, less than said predetermined rate, the ratio of heat absorbed in the superheating. step to the heat absorbed in vapor generation by withdrawing combustion gases from a position downstream gasfiow-wise ofv the superheating zone and thoroughly mixing the recirculated gases and the unrecirculated gases by introducing the recirculated gases into the flow of unrecirculated combustion gases passing from the combustion zone into the zone of vapor generation; causing the high temperature unrecirculated gases to sweep across such stratum of molten slag, the recirculated gases being introduced into the unrecirculated gases at a position substantially spaced from the. slag collecting zone; said introduction of the withdrawn gases including increasing the ratio of the rate of gas recirculation to the rate of vapor generation as the latter decreases to promote maintenance of the final superheat temperature at a predetermined value over an extendedrange of vapor generation rates.

2. In a method of generation and superheating of the high' pressure vapor; effecting substantially complete cornbustionof a slag forming fuel under highly turbulent conditions and at 'temperaturesabove the ash fusion temperature; separating the fused ash particles as a molten slag stratum in a slag removal zone; generating a high pressure vapor predominantly by the radiant transmission of heat from the combustion gases after they have left the combustion zone; superheating the generated vapor by convection heat transferred thereto from the cornbustion gases after they have left the zone of vapor generation; effecting optimum superheat at a predetermined rate of vapor generation; increasing at fractional loads, less than said rate, the ratio of heat absorbed in superheating to the heat absorbed in vapor generating by recirculating combustion gases from a zone beyond the superheating zone and introducing the recirculated gases at high velocity to the zone of unrecirculated gas flow at the gas inlet of the vapor generating zone and at a position substantially spaced from the slag removal zone; causing the high temperature unrecirculated gases to sweep said stratum of molten slag; and causing the recirculated gases to be thoroughly mixed with the unrecirculated gases by high velocity and counterflow jet action; said introduction of recirculated gases involving increasing the ratio of recirculated gas flow to the rate of vapor generation as the latter decreases to promote the maintenance of the final superheat temperature at a substantially constant value over a wide load range.

3. A steam generating and superheating unit comprising walls defining a vertically elongated gas-cooling chamher having a substantially horizontal floor, steam generating wall tubes of said chamber arranged to substantially reduce the temperature of the gases flowing therein, a bank of steam superheater tubes externally subject to gas flow from the upper portion of said chamber and arranged to be heated predominantly by convection, a cyclone furnace arranged with its major axis substantially horizontal and having a gas outlet opening into the lower portion of said gas cooling chamber, means for burning a slag-forming fuel in said cyclone furnace under average furnace temperatures above the slag fusion temperature, means forming a slag discharge outlet in said floor, means forming a slag discharge passage from said cyclone furnace opening into said gas cooling chamber below the level of said cyclone furnace gas outlet and above the level of said floor, means in the lower portion of said gas cooling chamber causing the fresh high temperature combustion gases from said cyclone furnace to sweep across said slag discharge outlet, and means for regulably increasing the convection superheating effect on said superheater tube bank including means for variably withdrawing a portion of the combustion gases at a point in the gas flow path beyond said convection superheater tube bank and returning said gases to the lower portion of said gas cooling chamber at a point in the gas flow path therein nearer to said cyclone furnace gas outlet than to said convection superheater and at a level substantially above the level of the floor so that the returning gases are separated from said floor slag discharge outlet by a layer of the higher temperature combustion gases.

4. A steam generating and superheating unit comprising walls defining a vertically elongated gas cooling furnace chamber having a substantially horizontal floor, said walls including vapor generating tubes of said chamber and arranged to substantially reduce the temperature of the gases flowing therein, some of said tubes defining a wall dividing said chamber into a primary furnace chamber and a secondary furnace chamber, a bank of steam superheater tubes externally subject to gas flow from the upper portion of secondary furnace chamber and arranged to be heated predominantly by convection, a cyclone furnace arranged with its major axis substantially horizontal and having a gas outlet opening into the primary furnace chamber, means for burning a slagforming fuel in said cyclone furnace under average furnace temperatures above the slag fusion temperature, means forming a slag discharge outlet in said floor, means forming a slag discharge passage from said c'yclone furnace opening into the primary furnace chamber below the level of said cyclone furnace gas outlet and above the level of said floor, the division wall extending into the lower portion of said gas cooling chamber to cause the fresh high temperaturecombustion gases from said cyclone furnace to sweep across said slag discharge outlet, and means for regulably increasing the convection superheating effect on said superheater tube bank including means for variably Withdrawing a portion of the combustion gases at a point in the gas flow path beyond said convection superheater tube bank and returning said gases to the lower portion of said gas cooling chamber at a point in the gas flow path therein nearer to said cyclone furnace gas outlet than to said convection superheater and at a level substantially above the level of the floor so that the returning gases are separated from said floor slag discharge outlet by a layer of the higher temperature combustion gases.

5. In a steam generating and superheating unit, means constituting a cyclone furnace burning a slag forming fuel while maintaining a normal mean combustion zone temperature greater than the slag fusion temperature, furnace chamber means including a primary furnace chamber receiving combustion gases from the cyclone furnace and constructed and arranged with a substantially horizontal floor receiving the fused slag from the cyclone furnace in a stratum near the bottom of the unit, the floor having a slag discharge opening therein, said fur nace chamber means also including a secondary furnace chamber receiving combustion gases from the primary furnace chamber, said furnace chamber means including vapor generating Wall tubes in which a predominant proportion of the total steam is generated as a result of the radiant transmission of heat thereto, the cyclone furnace and the furnace chamber means being constructed and arranged to cause the high temperature gases from the cyclone furnace to sweep said opening and the stratum of fused slag on said floor, a superheater receiving heat from the gases, and superheat control means including a gas recirculation system constructed and arranged to control superheat over a wide load range by increasing the percentage of gases recirculated as the rate of steam generation decreases, the gas recirculation system having a gas inlet communicating with the combustion gas flow beyond the superheater and having a gas outlet at a position beyond the cyclone-and remote from the slag stratum whereby gases at a temperature higher than the temperature of the introduced recirculated gases are interposed relative to said stratum and the position of recirculated gas entry, the furnace chamber means and the gas outlet of the gas recirculation system being arranged to cause the recirculated gases to meet the oncoming unrecirculated gases.

6. In a high pressure steam generating unit, means constituting a cyclone furnace burning a stream of slag forming fuel while maintaining a normal mean combustion zone temperature greater than the normal slag fusion temperature, the cyclone furnace being constructed to separately discharge molten slag and gaseous combustion products with the slag discharged at a position below the gases, gaseous furnace chamber means including a primary furnace chamber having a floor receiving the molten slag and normally covered with a stratum thereof, the floor having a slag discharge opening therein, the primary furnace chamber also receiving the combustion gases from the cyclone furnace, said furnace chamber means also including a secondary furnace chamber receiving the combustion gases from the primary furnace chamber, said furnace chamber means including steam generating wall tubes in which a predominant proportion of the total steam is generated by the direct transmission of radiant heat thereto, the cyclone furnace and the furnace chamber means being constructed and arranged to cause the high temperature gases from the cyclone furnace to sweep said opening and the stratum of fused slag on said floor,

a superh'eater receiving heat from the gases passing from the secondary chamber, and superheat control means including a gas recirculation system having a fan and fan inlet ductwork communicating with the gas flow beyond the super-heater, the gas recirculation system having fan outlet duct-work communicating with the flow of unrecirculated gases beyond the cyclone and ahead of at least a substantial part of the secondary chamber, the gas recirculation system constituting means for controlling superheat by increasing the percentage of introduced and recirculated gases as the rate of steam generation decreases, the furnace chamber means and the gas outlet of the gas recirculation system being arranged to cause the recirculated gases to meet the oncoming unrecirculated gases.

7. In the generation and superheating of high pressure vapor for use in the generation of power, cyclonically burning a stream of slag-forming fuel while maintaining a normal combustion zone temperature above the normal slag fusion temperature, discharging fused slag and gaseous combustion products from the combustion zone in such a manner that molten slag flows from the combustion zone separately from the gaseous combustion products and with the gases covering the fused slag, collecting the fused slag in a constantly discharging pool, generating vapor by the radiant transmission of heat to confined streams of a vaporizable liquid from the gaseous combustion products in a radiant heat zone beyond the fused slag zone, superheating the generated vapor by transmission of heat from the combustion gases beyond the generating zone, withdrawing a controlled percentage of the gases beyond the superheating zone, introducing the withdrawn gases into the radiant heat zone to meet the oncoming unrecirculated gases, the introduction of the withdrawn gases taking place in a portion of the radiant heat zone remote from said slag pool where higher temperature unrecirculated gases are interposed relative to said slag pool and the entering recirculated gases, and causing the high temperature combustion gases to sweep the fused slag pool before meeting the oncoming recirculated gases.

8. ln the generation and superheating of high pressure vapor, cyclonically elfecting the combustion of a slagforming fuel while maintaining a normal combustion zone temperature higher than the slag fusion temperature, separately discharging streams of fused slag and combustion gases from the combustion zone with the fused slag covered by the gases, collecting the fused slag in a constantly discharging pool, generating vapor by the radiant transmission of heat from the gases to confined streams of a vaporizable liquid, superheating the generated vapor by transmission of heat from the gases, withdrawing a controlled percentage of the gases after loss of heat therefrom in superheating, introducing the withdrawn gases into the radiant heat zone at a position spaced from the superheating zone and remote from the slag pool to meet the oncoming unrecirculated gases, mixing the introduced gases with the higher temperature gases ahead of the superheating zone, the introduction of the recirculated gases taking place where higher temperature gases are interposed relative to the slag pool and the entering recirculated gases, causing the high temperature combustion gases to sweep the slag pool before meeting the oncoming recirculated gases, and controlling superheat over a wide load range by varying the percentage of recirculated gas fiow, said variation of recirculated gas fiow involving the increasing of recirculated gas flow as the rate of vapor generation decreases.

9. A steam generating and superheating unit compriswalls defining a gas cooling chamber having a substantially horizontal fioor, steam generating tubes of said chamber arranged to substantially reduce the temperature of the gases flowing therein, a bank of steam superheater tubes externally subject to gas flow from said chamber and arranged to be heated predominantly by convection, a cyclone furnace having a gas outlet opening into the lower portion of said gas cooling chamber, means for burning a slag-forming fuel in said cyclone furnace under average furnace temperatures above the slag fusion temperature, means forming a slag discharge outlet in the floor of said gas cooling chamber, means forming a slag discharge passage from said cyclone furnace opening into said gas cooling chamber below the level of said cyclone furnace gas outlet and above the levelof said floor whereby a stratum of fused slag normally covers the floor, means in the lower portion of said gas cooling chamber causing the fresh high temperature combustion gases from said cyclone furnace to sweep across said slag discharge outlet, and superheat control means including a gas recirculation system for variably withdrawing a portion of the combustion gases at a point in the gas flow path beyond said convection superheater tube bankand returning said gases to the gas cooling chamber to meet the oncoming unrecirculated gases at a point in the gas flow path substantially spaced from the slag stratum so that the entering recirculated gases are separated from said floor slag discharge outlet by a layer of higher temperature combustion gases.

References Cited in the file of this patent UNITED STATES PATENTS 891,231 Cassens June 23, 1908 1,623,773 Bell Apr. 5, 1927 1,739,594 Jackson Dec. 17, 1929 1,789,401 Davy Jan. 20, 1931 1,837,713 Jacobus Dec. 22, 1931 1,933,020 Leamon Oct. 31, 1933 1,949,866 I-Iuet Mar. 6, 1934 2,100,190 Jackson Nov. 23, 1937 2,109,840 Gordon Mar. 1, 1938 2,229,643 DeBaufre Jan. 28, 1941 2,231,872 Bailey et al Feb. 18, 1941 2,357,301 Bailey et al. Sept. 5, 1944 2,421,761 Rowand et al. June 10, 1947 FOREIGN PATENTS 109,830 Australia Feb. 14, 1940 523,870 Great Britain July 24, 1940 

1. IN A METHOD OF VAPOR GENERATION AND VAPOR SUPERHEATING; EFFECTING THE COMBUSTION OF AN ASH BEARING FUEL AT A TEMPERATURE ABOVE THE FUSION TEMPERATURE OF THE ASH AND UNDER HIGHLY TURBULENT CONDITIONS; SEPARATING THE FUSED ASH PARTICLES FROM THE COMBUSTION GASES; COLLECTING THE FUSED ASH PARTICLES IN A STRATUM FOR CONTINUOUS REMOVAL AS MOLTEN SLAG; GENERATING A HIGH PRESSURE VAPOR AND SIMULTANEOUSLY COOLING THE COMBUSTION GASES PREDOMINANTLY BY THE RADIANT TRANSMISSION OF HEAT FROM THE GASES TO VAPORIZE CONFINED STREAMS OF A VAPORIZABLE LIQUID; SUPERHEATING THE GENERATED VAPOR BY CONVECTION HEAT TRANSFER FROM THE COMBUSTION GASES BEYOND THE ZONE OF VAPOR GENERATION; EFFECTING OPTIMUM SUPERHEAT AT A PREDETERMINED RATE OF VAPOR GENERATION; AND INCREASING, AT FRACTIONAL LOADS, LESS THEN SAID PREDETERMINED RATE, THE RATIO OF HEAT ABSORBED IN THE SUPERHEATING STEP TO THE HEAT ABSORBED IN VAPOR GENERATION BY WITHDRAWING COM- 